A variety of flat panel display devices are known including plasma and field emission display devices. references describing such devices include U.S. Pat. No. 4,857,799; U.S. Pat. No. 5,216,324; L. Branst and F. Pothoven, Semiconductor International, January 1996, p. 109; "The Grand Alliance in Flat Panels," Business Week, Aug. 28, 1995, p. 73; and C. Curtin, "The Field Emission Display: A New Flat Panel Technology," IDEC '91 pp. 12-15 (8/1991). The construction and operation of such devices vary but in general it is necessary to provide a high vacuum in a sealed interior volume between a base plate and a spaced face plate. The interior volume is defined and the vacuum maintained by an edge sealant which bonds the two plates together in spaced relationship. Current methods of vacuum sealing of flat panel displays require the use of solder glass frits as edge sealants. Such frits require long heating periods at 450-600.degree. C. to melt and fuse the frit with the glass panels of the display. There are, however, several problems associated with this method including long processing times, thermal degradation of critical components during the heat sealing operation, difficulty in maintaining micron size alignment tolerances under high heat conditions and compatibility of the frits with display glass.
While there are a number of alternative edge sealing techniques proposed, it is still desirable to obtain a commercially satisfactory alternative to the glass frit sealant. In particular, it is desirable to obtain a sealant which can produce a fixtured assembly at low temperature, which can withstand high temperature bake out, and which after bakeout, pumpout and closure of the pumpout port, can maintain a vacuum seal at pressures in the range of from about 10.sup.-3 Torr to about 10.sup.-9 Torr, depending on the specific application, without outgassing.
Unmodified alkenyloxystyrene adhesives are disclosed in U.S. Pat. Nos. 5,084,490 and 5,141,970 (McArdle et al) and in U.S. Pat. Nos. 4,543,397 and 4,732,956 (Woods et al). Poly(4-allyloxystyrene) and somewhat related polymers are described in Frechet, et al, "Imaging processes based on side-chain Modification of Polymers", ACS Symp. Ser.(1989), 381 (Eff. Radiat. High-Technol. Polym.), 155-71, and in Chem. Abst. 101:46315 (1984); 97:31277 (1982); 90:152895 (1979); 69:107140 (1968); and 67:11767 (1967). In all of these references the adhesives and other polymer systems are unfilled.
It is known that certain alkenyloxystyrene monomers, such as 4-allyloxystyrene, can be cationically photocured to produce a solid crosslinked polymer, which upon thermal baking will B-stage cure by a Claisen Rearrangement reaction to produce a polymeric material which has very high decomposition and glass transition temperatures. However, such monomers, and the adhesive compositions derived from these monomers, generally have very low viscosities compared to conventional adhesives and sealants. The low viscosity characteristics of these materials sometimes present processing difficulties related to adhesive "run-off" during the assembly of the components to be bonded together. In such circumstances, the adhesive bead, applied to one surface, flows beyond the intended bond or seal line area. This problem is particularly acute in those applications where assembly of the two substrates takes a relatively long time to complete after the application of the adhesive, such as is the case in the assembly of flat panel displays where precise alignment of the two substrates is a time consuming operation. The results of adhesive run-off include joint starvation with subsequent seal or adhesive failure, adhesive contamination and failure of contaminated components of the device to be sealed or bonded and increased processing costs related to adhesive wastage and clean-up.
A well known technique for overcoming such rheological problems with other monomer-based adhesive and sealant systems is to dissolve polymeric additives in the monomer composition, thus increasing the viscosity and minimizing or preventing the adhesive run-off during assembly. However, it has been found that conventional polymeric thickeners such as polystyrene do not possess adequate thermal resistance properties to be useful in alkenyloxystyrene compositions intended for applications with high-temperature and/or high-vacuum, low-outgassing requirements. Furthermore, as illustrated in the examples below, the corresponding polymers prepared by the conventional polymerization of 4-allyloxystyrene monomer have been found to be unsuitable due either to chemical instability (cationically polymerized polymer, see Example 4, composition C) or insolubility (free radically polymerized polymer, see Example 3).
Polymerization of 4-allyloxystyrene by means of free radical initiators is reported in J. Frechet et al, in ACS Symp. Ser. 381 (Eff. Radiat. High-Technol. Polym.), 155-71, (1989), and in Chem. Abst., 69:107140 (1968) abstracting M. Kato et al, J. Polym. Sci., Part A-1, 6(11), 2993-3006 (1968). JP 59034532 (abstract), reportedly describes an anionically polymerized 4-allyloxystyrene polymer. Such material would be expected to inhibit cationic curing and therefore would be unsuitable as thickeners for cationically curable compositions.